I have a theory that Reina is from the Hachijo family, the Mishimas do not have the Devil Gene, the Devil Gene originated from the Hachijo, maybe she is not even Heihachi's daughter, but if she is his daughter, Kazumi cannot be the mother, so Heihachi may have had a love affair with another woman from the Hachijo family, perhaps a cousin of Kazumi, or Kazumi's sister.
I have a theory that Reina is from the Hachijo family, the Mishimas do not have the Devil Gene, the Devil Gene originated from the Hachijo, maybe she is not even Heihachi's daughter, but if she is his daughter, Kazumi cannot be the mother, so Heihachi may have had a love affair with another woman from the Hachijo family, perhaps a cousin of Kazumi, or Kazumi's sister.